Photoemission study of ZnO nanocrystals: Thermal annealing in UHV and induced band bending

► High resolution XPS was used to study ZnO nanocrystals distributed on SiO2/Si. ► The optimal cleaning temperature for ZnO nanocrystals in UHV is found to be 650 °C. ► A large downward band bending of 1.4 eV is observed after annealing at 750 °C. ► Band structure changes related to chemical changes and point defects are discussed. ► The concentrations of both oxygen and zinc vacancies affect the band bending.